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#include <bits/stdc++.h>
#define REP(i,n) for (int _n=(n), i=0;i<_n;++i)
#define FOR(i,a,b) for (int i=(a),_b=(b);i<=_b;++i)
#define FORD(i,a,b) for (int i=(a),_b=(b);i>=_b;--i)
#define TRACE(x) std::cerr << "TRACE(" #x ")" << std::endl;
#define DEBUG(x) std::cerr << #x << " = " << (x) << std::endl;

void init_io() {
  std::cin.tie(nullptr);
  std::ios::sync_with_stdio(false);
}

int main() {
  init_io();

  int n, want;
  std::cin >> n >> want;

  std::vector<bool> seen(n);
  std::int64_t cost = 0;
  int have = 0;

  REP(i, n) {
    if (have == want) break;
    int brand; std::cin >> brand;
    --brand;
    if (!seen[brand]) {
      seen[brand] = true;
      cost += i - have;
      ++have;
    }
  }

  if (have < want) cost = -1;

  std::cout << cost << '\n';
}